For example, many in-line screw type injection molding machines of electrically driven types according to the background art use the following configuration. That is, a timing belt is extended between a driving pulley fixed to an output shaft of a metering electric motor and a driven pulley formed integrally with or fixed to a rotary connector to which a base end portion of a screw is fixed. Rotation of the metering electric motor is transmitted through a rotation transmitting mechanism constituted by the pulleys and the belt to the rotary connector retained rotatably on a linear motion block to be driven linearly by a driving force of an injecting electric motor. Thus, the screw integrated with the rotary connector can be rotated. When such a configuration is used, a general AC servomotor having an output shaft in its center can be used as the metering electric motor. However, a reduction mechanism has to be built by the rotation transmitting mechanism constituted by the pulleys and the belt. Thus, the number of parts increases to hinder space saving of the machine. In addition, since it is necessary to rotate the driven pulley whose diameter is comparatively large, the rotational inertia increases to limit the improvement of the transient response performance of rotation transmission unavoidably.
An injection molding machine using an internally hollow built-in type motor (direct coupling type motor) as the metering electric motor in order to reduce the rotational inertia to thereby improve the transient response performance of rotation transmission has been known, for example, in JP-A-08-039631 (Patent Document 1). The background-art technique in Patent Document 1 discloses a configuration in which a spline shaft formed integrally with a screw is spline-connected directly to a rotor of a built-in type motor as a metering electric servomotor so as to make the built-in type motor drive and rotate the screw. According to the invention proposed in Patent Document 1, there is provided a single built-in type motor serving as both a metering electric motor and an injecting electric servomotor, in which a ball screw or a spline shaft portion of a ball screw/spline shaft unit integrated with the screw is alternatively selected and rotated by a clutch so that the screw can be alternatively selectively rotated or moved linearly.
In the background-art technique in Patent Document 1, however, the ball screw is formed behind the screw and the spline shaft is formed behind the ball screw, so that the entire length of the machine becomes long. In addition, since the spline shaft is spline-connected to the rotor of the built-in type motor, constituent elements for the spline connection are required to thereby result in labor increase for the attachment.
According to the invention proposed in Patent Document 1, a single built-in type motor serves as both the metering electric motor and the injecting electric servomotor. It is therefore necessary to provide two clutches and a spline shaft connection mechanism. Thus, there is a problem that the structure is complicated and the assembly is troublesome. In addition, while the screw is rotated, axial pressure cannot be applied to the screw by the motor. Thus, there is another problem that back pressure cannot be applied in a metering step.
In an injection molding machine using a built-in type motor as a metering electric servomotor, it can be therefore regarded as rational that a rotary connector fixing and holding a base end portion of a screw is fixed to a cylindrical rotor rotating inside a cylindrical stator so as to transmit rotation of the metering built-in type motor directly to the screw and to thereby rotate the screw. In addition, it can be regarded as general that a linear motion portion of a ball screw mechanism for converting rotation of an injecting electric motor into linear motion is connected and fixed to a linear motion block which is mounted with the metering built-in type motor and which moves forward/backward, so that the linear motion block is driven linearly by the driving force of the injecting electric motor to thereby move the screw linearly.    Patent Document 1: JP-A-08-039631